In many electric circuits and devices there is a need for switching means which have the maximum permissible voltage in an open state, which have low resistance in a closed state and which provide galvanic insulation between the control circuits and main circuits and between different main circuits in the same device. An example of equipment in which switching means of these kind are required is telephone equipment. In typical equipment of this kind, electromechanical relays are used as switching means, and for each telephone line arriving at a telephone station, a plurality of such relays are required, for example for connection of a ringing generator, for disconnection and connection in connection with trouble shooting, and the like. Electromechanical relays bulky, are relatively expensive, the mechanical contacts give limited reliability, for example, because of fouling of contacts, and the relays require a relatively extensive assembly and connection work during installation in the equipment in question.
From the article "Monolithic Capacitor-Coupled Gate Input High Voltage SOS/CMOS Driver Array" (H Sakuma and K Hirata) in Proceedings of the IEEE Custom Integrated Circuit Conference (IEEE CICC), 1984, pages 564-568, a unidirectional MOS switch is previously known. This switch exhibits a couple of highly doped contact regions with two intermediate, weakly doped voltage absorbing regions. Between these regions and one of the contact regions a MOS structure is formed, and with the aid of the control electrode of this structure the switch can be controlled between closed and open states. As previously mentioned, this switch is unidirectional, in that it is only able to absorb voltage in one direction. In a large number of pieces of equipment, a switching member of the current type is required to be bidirectional, that is, to be capable of absorbing voltages of both polarities and, in addition, it is required to carry current in both directions. It would be possible to achieve a bidirectional switch by series-connecting two opposed members of the type described in the above-mentioned article. The on-state resistance of such a series connection would inevitably be high, and to reduce this resistance to acceptable values, a relatively large area would be required.